In commonly owned copending, allowed application U.S. Ser. No. 354,859, now U.S. Pat. No. 4,424,191, filed on Mar. 4, 1982, by Raymond F. Jakubowicz, entitled "Analyzer Featuring Loading and Unloading Means for a Storage Chamber, and Common Drive Means", corresponding to European Patent Publication No. 88,601 published on Sept. 14, 1983, there is described a simplified analyzer for use in small offices, such as doctor's offices. Incubated test elements are pushed through a photometer station, one at a time, by a pusher blade that has a black reference and a white reference coated on the underside of the pusher blade. Because the purpose of the analyzer is to provide an inexpensive way to measure the analytes body liquids using test elements containing all the necessary reagents preincorporated therein, such reference coatings are inexpensively made. That is, the black reference may not be an ideal black (perfectly absorptive), and the white reference may not be an ideal white (perfectly reflective). In such a case, it is possible for a test element to produce a reflection density that is darker than the "black" reference, or lighter than the "white" reference.
The most conventional calculation of reflection density D.sub.R follows the equation EQU D.sub.R =-log.sub.10 [(A/D.sub.sample -A/D.sub.black ref)/(A/D.sub.white ref -A/D.sub.black ref)] (1)
where A/D represents the analog-to-digital electrical signal generated by the reflectance of either the sample (usually the test element), black reference, or white reference. It will be readily apparent that the possible case noted above of a blacker or a whiter test element (than the reference) will throw off the calibration curve, at best. At worst, in the case of a blacker test element, it produces a negative reflectance, an artificial concept. It can be shown that, to correct for such non-ideality in the black and white references equation (1) should be modified as follows: ##EQU1## wherein R.sub.sample.sup.uncorr is exactly the argument of the log of equation (1), that is: EQU R.sub.sample.sup.uncorr =(A/D.sub.sample =A/D.sub.black ref)/(A/D.sub.white ref -A/D.sub.black ref); (2)
and R.sub.white.sup.effective and R.sub.black.sup.effective are the effective reflectances of the white reference coating and of the black reference coating, respectively. Thus, R.sup.uncorr is adjusted (equation 1a)) to become the corrected reflectance by, first, multiplying it with (R.sub.white.sup.effective -R.sub.black.sup.effective), and then adding to the product term R.sub.black.sup.effective.
Such effective reflectances are determined using a referee photometer or reflectometer wherein the black and white primary references are carefully (and thus, more costly) selected to be blacker and whiter, respectively, than the blackest and whitest sample that is likely to be read thereon. In other words, the referee reflectometer is selected to have substantially ideal black and white primary references. A representative example of such an instrument is the reflectometer obtained from Zeiss Company under the trademark "Zeiss DMC-26".
The conventional method of calibration by ascertaining the values of R.sub.white.sup.effective and R.sub.black.sup.effective as the corrective factors for equation (1a) above, has been to remove the non-ideal black and white references from the inexpensive analyzer, and read tham as intrinsic reflectances on the referee reflectometer. This, however, ignores an important factor about the location of such black and white reference coatings. As described in the aforesaid application, the black and white reference coatings are located in the test reflectometer at a position that is optically different from the detection position of the detectable portion of the test element carrying the liquid sample. That is, the reference coatings are positioned for detection displaced from the detection position of the test elements, thus producing a variation in the length of the optical path. Although the displacement of such two positions can be made to be as small as possible, there is still about 0.5 mm difference between the two. Such displacement can be essentially eliminated by requiring the operator to send through the black and white references as test elements every time a sample is being read. However, this alternative has several disadvantages. One is that repeating a "run" of the black and white reference as a special kind of test element along with every sample test element runs the risk of the reference "test elements" being lost since they would not be permanent parts of the analyzer. Another is that positioning the black and white references at the detection location of the test elements prevents the apparent reflectance of such references from being altered or corrected by changing the displacement distance. Instead, the intrinsic reflectance has to be modified by a chemical or structural change to the coating itself. Finally, the analyzer of the aforesaid application requires the photometer to contact the test elements conveyed through it. If the black and white references were also read by contact, a transparent protective, and expensive, coating would have to be added to prevent scratching.
Notwithstanding the advantage of such a displacement, displacement has been objectionable because the apparent reflectance of the black or white reference is altered from what it would have been if the references were located the same distance from the light source as were the test elements, as is well known. Such alterations in apparent reflectance can produce an error in detected reflection density which is as much as 50%-70% in the conventional method.